High purity silica glass material has become the material of choice for lens elements used in high-resolution deep UV and vacuum UV projection lithography devices. Excimer laser beams are typically used in those lithography devices. It is known that presence of H2 molecules in the glass is conducive, and indeed sometimes necessary, for the laser damage resistance of silica glass when used at about 248 and 193 nm. Some as-manufactured silica glasses do not contain molecular H2 per se. Thus those skilled in the art have proposed various processes for adding H2 molecules into consolidated silica glass, which is called hydrogen loading.
Teachings regarding the relationship between the distribution of H2 in the silica glass and its optical performance at about 193 nm in terms of LIWFD, refractive index and refractive index homogeneity, and the like, in the prior art, if any, is scant. They provide vague at best, and contradictory at worst, guidance as to the development of a most efficient and effective hydrogen loading process in the process of making silica glass material suitable for use in the refractive lens element of modern lithography devices operating in deep and vacuum UV regions. They mostly teach hydrogen loading at high pressure, and sometimes at high temperature, which are undesirable. Moreover, they invariable require long hydrogen loading time to achieve the stated hydrogen concentration levels. Since hydrogen loading is a diffusion process, and it occurs at a very slow diffusivity rate, it has become one of the most time-consuming and expensive steps in the silica glass manufacture process. Reducing the loading time while achieving acceptable and excellent optical performance is a serious challenge in the art up until the advent of the present invention.
Therefore, there is a genuine need of an efficient and effective process for hydrogen-loading silica glass material to produce material that meets the requirements of lithographic applications operating at a wavelength below about 300 nm, especially in the deep UV and vacuum UV regions, particularly at about 248 nm and 193 nm, and silica glass materials loaded with hydrogen at levels that generate desired optical properties thereof.
The present invention satisfies this long standing need.